Black mustard is probably endemic in the Southern Mediterranean region, but has been cultivated since thousands of years; therefore, numerous cultivars are found.
Botanically different, though of equal use in the kitchen, are the Sarepta mustard or Romanian Brown Mustard (Br. juncea) from Eastern Europe and the Indian Brown Mustard (Br. integrifolia or Br. juncea, a fertile hybrid from Br. nigra and Br. campestris) from India and Central Asia. Of all three species, the latter is probably most commonly sold in the West.
Although the pungency of black mustard is slightly stronger than that of brown mustard, black mustard is hardly planted in Europe anymore, and brown mustard is the dominating quality on the European market. The reason is that brown mustard, unlike black mustard, can be harvested by machines which make production much cheaper in countries where working force is expensive.